Today's problem is that I mounted a second oil pressure (electrical not mechanical) gauge on the fire wall. I grounded the gauge and then attached the sending wire to the sender mounted a the rear of the block a SBC 350. This made the gauge in the dash cut out. I did this three times, sure enough all three times the dash gauge stopped working. Any suggestions on how to get them both to work at the same time? Would splicing the two sending wires together first and then to the sender work? This idea seemed to me as it would not make any difference.

Most electrical oil pressure guages are wired with the 12 volt feed to the guage. The sender is controlling the guage of the ground side.
By connecting the hot wire to the sender you loaded down the circuit through the sender.
This likely damaged the sender by having a large current flow through it to ground.
Normally a sender will only handle .15 amps +-.
Replace it to be sure.

VDO uses a sender and guage combination that, as the pressure increases the resistance of the sender increases.
For example, With the engine off (no pressure) the sender is at 10 ohms.
With the engine running with 40 psi of pressure the sender is at 105 ohms.
With the engine running with 60 psi of pressure the sender is at 152 ohms.

All this means is the sender must be matched to the guage.

It's possible that you could run (2) guages from (1) sender if the guages are exactly the same but I've never tried it.

vicrod

2 guages= 2 block senders=2 independant readings...use a "T" fitting for the second sender

Doc here,

If you lose both functions when you parallel them into the same sender, you are probably back-feeding resistance from one gauge to the other..

(the gauge thinks because there is a high resistance out there, there is no sender..therefore it pegs out ..)

IF both gauges are the same resistance requirement for the sender, (identical electrically) Then you can run a sender wire from both to a point where the can both meet, and place a diode on each wire from the Gauge to the Anode of the diode..Tie the Cathode Side of the diodes together and run a single wire to the sender unit..



In this manner, it will isolate the gauges from "Back loading " each other causing the loss of function.

Option 2..

If the Firewall gauge is to be used only while working under the hood...you could install a double pole , single throw switch at the fire wall gauge, and switch the sender wire between the inside gauge to the outside gauge..(since you can't be in both places at once..) Center wiper of the switch to the sender, up position to the inside gauge, down position to the outside gauge..

Option 3...

You could find a single slide, double pole switch ...

(like a door post dome light switch, except open = on #1 closed = on #2.. 3 contacts)

Wire it so the center wiper is the sender, and the up position is the firewall gauge..the down position is the inside gauge..

mount it on the fender lip, or radiator support, so that when properly adjusted, as you open the hood, the gauge inside the car turns off, and the fire wall gauge turns on..Visa versa when closing the hood..

you can find a switch like this at any local Car alarm shop..just be sure it is an isolated from ground switch..(3 terminal) ..If you cant find that...on to..

Option 3..

get a door post dome light switch (single contact) and a relay, wire the relay coil to a switched power source, and the coil ground to the switch..

The normally closed contact will be the inside gauge, and the center wiper will be the sender..the normally open contact will be the fire wall gauge..(this will work the same as the switch in option 2..)

Doc

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Option 1; When I parrallel the second or firewall gauge to the sender the needle in the dash gauge goes back to zero. The firewall gauge then works fine. Secondly the diode thing, what do I ask for when I go to the (Auto Parts Store?). That is are all diodes the same? If not what type do I ask for.

Doc here,

Go to Radio Shack, ask for a pair of 1N4001's , that should work fine for your application.

Doc

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2000jack,
Doc's diode plan is the simplest,
I would use a rocker switch, keeps the engine guage in "new condition" longer (to compare/verify the dash guage reading accuracy)

advantage of seperate senders and guages (T fitting) for any analog electric guage is to verify the guage/sender calibration curve is correct for both (both read 20 independently, both read 40, etc.)

better(?) indicator for the "made in China" inaccurate boards that's getting worse on car parts....any solid state board can go whacko still work but with a wrong result-reading and oil pressure is critical!

Beach Boys- Little Deuce Coupe. There was a extra set of gages on the firewall, just to be a little 50's-60's style on my 29 sedan, that is why I decided to add the gages. So far I got a vacuum gauge on the block, behind the alt., fuel pressure gauge on the fuel block, a tach mounted in front of the passenger's side of the engine, where the PS would be located. The fire wall has an oil pressure, oil temp, and trans temp in an aluminum panel. Yeah probably got carried, but that is why I bought it 60% done so can beat my head on the wall when things don't work out the way I plan them! As I do want both to work at the same time. I like your idea of the extra switch, no sense in running the gauge when I am on a trip driving on the interstate. Also docvette seems to come through every time to solve rodders questions. KUDOS for docvette!!!!

just to be clear...not a on/off switch, use a power to "A" or "B", center= both off rocker switch (radio shak)...only one guage is on

you want both on, use the diode

If I wanted to hook up a second Tachometer would I use this method as describe above with the diodes or could both sending wires be tied in directly at the coil? Thanks a lot

2 tach's don't need diodes

tach is just a ignition pulse "counter" compared to time elapsed convertor (then compared to where you set the dip switch, 4-6-8 cylinders for correct reading) (doesn't measure V or ma as a analog scale to convert)

ex: hook your car coil up to a repair shop "engine analyzer" or a dwell tach meter...both read fine...."each" is counting the same pulse

Thanks! I had bought a used tach and it did not work, so I thought maybe it was for the reason the oil gages did not work. I bought a new one, but I have not hook it up yet and I did not want to mess it up. Actually I have used a tach box years ago, as you said.

check the dip switch for good/bad with your volt meter on ohms scale, and/or broken wire at the dip switch on the busted used tach before you throw it away cause the board is dead

Doc,

Thanks for posting this info.

It had never occured to me to put gauges under the hood, but this could be a fantastic idea. Sure would make troubleshooting problems easier.

An offshoot of this might be to build a remote gauge pod that would plug into a harness under the hood. Then, if you're troubleshooting, you could plug it in and get the full array of in-dash gauges while under the hood and not have to actually mount a set there.

Certainly, you can get a lot of this info with a scan tool if you have a computer, but if you don't...